This invention relates to internal combustion engines. More particularly, this invention relates to engine lubricant level sensors, such as low oil sensors.
Several types of lubricant level sensors are known. In a float-type sensor, a contact is attached to a float which rides on the surface of the lubricant reservoir or crankcase. When the lubricant level sufficiently drops, electrical contact is made between the contact on the float and ground, so that ignition pulses are shorted to ground and the engine is stopped.
There are several problems with such float-type low oil sensors. For example, these sensors may give false readings when a splash-type lubrication system is used in a small internal combustion engine. In a splash-type lubrication system, a slinger gear or paddle splashes lubricant throughout the engine housing while the engine is running. Shortly after the engine has started, much of the lubricating fluid has been splashed throughout the engine housing, so that the actual level in the lubricant reservoir is very low, even though the overall engine lubrication level is satisfactory. As a result, the float switch may be erroneously closed even though the amount of lubricant in the engine is sufficient. To avoid such false readings, it is known to use a timer circuit to delay the indication of a low oil condition for a preset period of time after engine starting. After the time delay period has passed, the lubricant level sensor operates in a normal manner.
Another problem with low lubricant sensors is that the low lubricant level switch tends to oscillate or bounce when the lubricant level in the crankcase is fluctuating. Such fluctuations may occur during normal engine operation, particularly in splash-type lubrication systems, since the amount of lubricant actually in the crankcase is not necessarily a correct indication of the amount of lubricant in the engine. The switch may also oscillate or bounce if the engine is being tilted, which occurs, for example, when the engine is applied to a lawnmower that is moving over an uneven terrain. As a result, the engine may misfire since some of ignition pulses are being grounded and others are not. To overcome this problem, it is known to impose a delay period after the low lubricant level switch closes before an indication is provided of a low lubricant level condition. For example, see U.S. Pat. No. 3,886,517 issued May 27, 1995 to O. H. Taken et al. and U.S. Pat. No. 4,838,082 issued Jun. 13, 1989 to McCoy et al.
A significant disadvantage of these time delay approaches is that they typically require relatively complicated and expensive circuitry, which may not be feasible for a lower cost internal combustion engine.